Modulation system



Nov. 21, 1939. Av s. GANO MODULATION SYSTEM Filed July 6. 19s? INVENTOR ALFRED S. GANO ATTORNEY Patented Nov. 21 1939 Q PATENT OFFICE MODULATIONPISVYSTEM Alfred. S. Gano, New YorkQN. Y., assignor to Associated Press,,New York, N. Y., a corporation of New York Application July 6, 1937 Serial No. 151,988

, zClaims. K01. 179-1715) This invention relates to picture transmission systems.

An object of the invention is to provide a balanced modulator which suppresses the signal Q 'o' and is capable of varying the amplitude of a carrier P iinerally over a wide, range when the signal available from a photoelectric cell is small.

In modulator systems, such as those used for 10 speech transmission and for radio operation,

there is usually a great difference in thefrequency of the signal and the carrier, making it possible to filter out the signal by conventional filter systems, but in certain picture transmission I systems where the frequency of the signal is very close to that of the carrier, this method of eliminating the signal is not desirable. The present invention contemplates a signal from zero to 1200 cycles which has been found suitable'for scanning purposes, and a carrier of 2400 cycles, the latter'being adapted for use in connection with leased line facilities. now available.

Another object of the invention is to provide a simple and inexpensive circuit arrangement particularly suitable for portable'picture transmission apparatus.

Still another object is to eliminate the light valve commonly used in picture transmission systems which is a delicate and expensive piece of apparatus requiring frequent adjustment.

' The invention will now be described with the aid of the accompanying drawings of which there is but one sheet.

The figure shows a schematic wiring diagram of the circuits used in connection with the invention.

Referring now to the drawing:

Reference character I indicates part of an optical system through which rays of light reflected from a picture on a scanning drum are concentrated on the plate 2 of a photoelectric cell 3 according to well known practice. In the present case, a small point of light having a constant intensity is focussed on the picture and is reflected from the picture through a suitable optical system I onto the plate of the cell, the light varying in intensity according to the density of the shading of the elemental areas of the picture being scanned.

50 Cathode 4 of the photoelectric cell is connected to one side of a source of polarizing potential 5, the opposite side of which is connected through resistance Rl to the plate of the photoelectric cell.

55 Plate 2 is connected to control grids 6 and 1 terminals of primary II of a push-pull output of. vacuum tubes 8 and 9 respectively. In the present embodiment of the invention these tubes are of a type known to the trade as No. 77 pentodes. I Battery Ill, shunted by variable resistance ll, 5 one'side of which is connected to battery 5, is connected by the slider of resistance II to cathodes 23 and I2 of tubes 8 and 9 respectively. The cathodes are also connected respectively to suppressor grids I3 and I4. 10

Battery I5, the negative side of which is grounded at -Eb which is a common ground for the circuit, has its positive side connected through R2 to screen grid l6 of tube 8 and through resistance R3 to screen grid ll of tube 9. Y 'Plate potential from +Eb is supplied to plate It of'tube 8 through resistance R8 and one section.l9 of the primaryv of push pull transformer 20. In the same manner plate voltage is supplied to plate 2| through resistance R9 and the second section 22 of the primary of transformer 20.

One side of the primary winding 22 is connected through condenser C3, resistance R6, condenser Cl and resistance' R4 to screen grid 5 25 oftube 8.- In the same manner primary winding I9 is connected through condenser C4, re

- sistance R1, condenser 02, and resistance R5 to screen grid ll of tube 9. "An amplifier, which in the present embodi- 3 ment includes two triodes and 3!, is controlled by tubes 8 and 9. The secondary 32 of transformer 2B is connected to grid 33 of tube 30 and the opposite terminal of the secondary to grid 35 of tube 3|. Cathodes 3e and 31 are grounded. C battery 38 provides biasing current for grids Wand-35.

Plates 39 and 4B- are connected to the outer transformer 42, plate voltage being supplied from 40 -|-Eb to the midpoint of the primary. Secondary 43 is supplied with suitable leads for externalconnections.

An oscillator 44 having one output lead con- 45 nected between condenser Cl and resistance R6, and a second lead connected between condenser '02 and resistance R'l provides the carrier P for the system.

The resistance of the photoelectric cell varies of the grids of the two input tubes. For transmission purposes, it is desirable to eliminate as far as possible, the signal Q in the output of tubes 353 and Si in order that there may be no interference with the modulated signal PiQ.

Signal Q is a direct current pulsating at a rate from zero to about 1200 cycles depending on the picture being scanned. This signal goes to control grids 5 and I and causes a current of like polarity to flow in both halves of the primary of the push-pull transformer. Since these currents are of equal magnitude and flow in 01)- posite directions from the common junction point of the primary sections [9 and 22 of the transformer they will induce no voltage in the secondary sections 32 of the transformer. The lower frequencies of signal Q are not suited for transmission over communication lines, so therefore a carrier P from a suitable oscillator 44 is introduced into the circuit at P at about 2400 cycles or other suitable frequency.

The carrier feeds to the screen grid and the plate of tubes 8 and 9, and the polarity fed to the plate of one tube is constantly the reverse of that fed to the plate of the other, so that the current flowing in section IQ of the transformer would be so polarized in respect to that in section 22 as to be mutually aiding and thus induce a voltage in the secondary sections 32.

The circuits are so adjusted that when cell 3 is dark, or exposed to a minimum amount of light, such as occurs when the darkest part of a picture is scanned, no appreciable Q or P appears in the output of the circuit.

The values of the various resistances and condensers vary according to the kind of tube used, voltage conditions, and the particular application to which the circuit is to beput. However, once the resistance and condenser values are decided upon for a certain type tube, there is little likelihood that these values will require changing unless circuit conditions are altered. An exception to this statement is that the resistance H for varying the C bias for tubes 8 and 9 is arranged to be changed as required.

It will be noted that thecarrier voltage applied to the anode of either tube 8 or 9 always has a difference in phase of degrees with that of the screen grid of the same tube, and the purpose of the condenser and resistance network through which the oscillator is connected to these tubes is to compensate for any electrical characteristics of the tubes or circuits which would cause any slight variation of this phase difference.

With cell 3 dark, the amount of carrier fed to the plates is adjusted so that it neutralizes the carrier flowing in the screen grid circuit. In this manner the carrier can be suppressed in the output to almost any desired point for particular signal voltage conditions.

Assuming the circuit to be so adjusted, then when lighter parts of the picture are scanned the resistance of cell 3 will drop accordingly and the voltage impressed on grids 6 and I will vary, consequently the balance previously obtained while cell 3 was dark will be upset and a carrier from oscillator M modulated according to the changing voltage on the control grids Will flow through the primary windings of transformer 20' inducing a corresponding current in secondary 32which in turn controls amplifier tubes 30 and 3|.

What is claimed .is:

1. In a modulator for picture transmission systems for the purpose of obtaining a high ratio of side band output to carrier output and at the same time suppressing the signal in the output, a source of signal current, a pair of vacuum tubes, each having an anode, a cathode and a plurality of grids, including a screen grid; a circuit connecting a grid in each tube to the same pole of said source of signal current, a circuit connecting the opposite pole of said signal source to the cathodes of said tubes; a pair of circuits each connecting a screen gridof one tube with the anode of the opposite tube, an oscillator having opposite output terminals connected to each of the last mentionedcircuits, an output circuit connecting said anodes, and means including resistances and condensers in said pair. of circuits balanced so that substantially no oscillator current flows in said output circuit.

'2. In a modulator for picture transmission systems for the purpose of obtaining a high ratio of side band output to carrier output and at the same time suppressing the signal in the output, asource of signal current, a pair of vacuum tubes, each having an anode, a cathodeand a plurality of grids, including a screen grid; a circuit connecting a grid in each tube to the same pole of said source of signal current, a circuit connecting the opposite pole of said signal source to the oathodes of said tubes; a pair of circuits each connecting a screen grid of one tube with the anode of the opposite tube, an oscillator for providing a carrier voltage and having opposite output terminals connected to each of the last mentioned circuits, an output circuit connecting said anodes, and means including resistances and condensers in said pair of circuits of such value that the carrier applied to the respective anode is 180 degrees .out of phase with that applied to the associated grid and of such magnitude that the carrier voltage applied to the grids is substantially balanced out in .the output from said anodes under predetermined signal voltage conditions.

ALFRED S. GANO. 

